FIG. 2A illustrates the typical optical communication system. In a transmitter 20, light from a source 22 is modulated with the modulator 24. The modulated light is transmitted over an optical link such as a fiber optic link to a receiver 26. At the receiver, the light is detected with a detector 28 and the modulated signal is electronically modulated in a demodulator 30.
Typically the transmitted light is intensity modulated and received light is detected directly (IM-DD). Both digital and analog optical communication systems use intensity modulation. IM-DD offers a number of practical advantages that have made it the popular choice for network designers. These are:    1) Bright sources (e.g. semiconductor lasers) are readily available that can be modulated directly and rapidly (>25 GHz).    2) Receivers are easy to design and inexpensive to build and can operate at high data rates (demonstrations of >40 Gbs)    3) Optical amplifiers are available (especially at 1550 nm) that allow signals to be boosted and shot noise performance can be readily achieved.
However, IM-DD is not without important limitations. Perhaps the most important limitation is channel non-linearity. This is especially a problem in fiber optic links where non-linear processes such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), cross-phase modulation (XPM), self phase modulation (SPM), and four-wave mixing (FWM) place a fundamental limit the amount of power that can be sent through optical fiber. XPM and SPM causes amplitude modulation to be converted to phase modulation, which leads to: (1) non-linear dispersion, or pulse broadening, (2) pulse jitter and (3) cross talk between multiple wavelength channels in a wavelength division multiplexed network (WDM). Typically, the maximum practical power that can travel inside a single mode fiber before limitations due to non-linear processes are observed is ˜8–10 dBm.
Another important limitation with current IM-DD systems is low spectral efficiency. Even in dense WDM systems, typical 10-Gbs links have channel spacing of 50 GHz and higher, leading to a spectral efficiency of 0.2 b/s/Hz. This is to contrasted with RF wireless links that have 2 b/s/Hz and higher. The reason for the poor efficiency is the difficulty in fabricating and maintaining filters with narrow enough line widths. Multiplexing multiple IM-DD at a single wavelength is very difficult (e.g. code division multiple access CDMA). Future fiber optic links are expected to achieve bandwidths of greater than 40 Gbs and higher, and will involve significant (costly) upgrades to existing technology. Methods to increase the spectral efficiency of existing slower speed links to achieve the high data rates is therefore of immediate interest.
These reasons suggest that a method that avoids the use of amplitude modulated signals to communicate information, and which can increase spectral efficiency, would be of significant interest. One such approach is coherent communication.
Coherent detection (hence “coherent communication”) typically refers to the use of a strong local oscillator (LO, which is typically a laser in optical communication) to boost a weak signal in order to enable its detection over detector thermal noise. Typically, the LO is mixed with the weak signal on a non-linear detection element (e.g. photodetector in optics) and the beat frequency between the LO and the signal is measured. The underlying physics exploited by coherent detection is the electromagnetic interference between the signal and the LO. In order for this interference to produce a measurable signal, most communication applications to date require the LO to be “coherent,” which is the same as requiring the optical phase/frequency to be well defined (typically constant) over the length of measurement.
Coherent detection is contrasted by direct detection, which is based on modulating the amplitude of the transmitted electromagnetic radiation and detecting the intensity of the received signal. Coherent communication requires the manipulation of the phase of the electromagnetic radiation, whereas direct detection only requires the modulation of the amplitude. Angle-modulation (PM, phase modulation, and FM, or frequency modulation) offers a number of advantages over IM-DD. These include: (1) higher sensitivity per photon, (2) a larger dynamic range, (3) better performance in a clutter environment, (4) insensitivity to various types of non-linearity in the channel, and (5) increased spectral efficiency (i.e. more bits transmitted per signal bandwidth).
With the widespread use of erbium amplifiers to boost weak signals, the higher sensitivity offered by coherent systems has largely disappeared. Therefore, sensitivity is not a critical decision parameter in the choice between IM-DD and coherent receivers. However, what is important is that angle-modulated systems have much larger dynamic range over which signal linearity can be maintained. This can be particularly important to analog communication links where data high SNR as well as high dynamic range is of critical importance.
Although usually not considered as a principal feature, angle-modulate systems have good robustness against non-linearity, both in the channel as well as in the receiver. This feature will be stressed in some detail below. An important feature of an angle-modulated system is that the carrier is a constant amplitude signal since the phase modulation imparted on the carrier does not typically lead to an amplitude modulation. Therefore problems from XPM and SPM are significantly mitigated, especially in single channel systems. Additionally, it is well known that angle modulated systems have significantly reduced sensitivity to receiver non-linearity as compared with IM-DD systems. By introducing a frequency offset in the LO with respect to the signal, receiver non-linearity in FM/PM systems can be separated and filtered in the frequency domain.
In fact, it is the latter feature that often is emphasized as one of the principal features of coherent links. By using multiple frequency and/or phase offsets, coherent transmission offers the most simple and straightforward approach to multiplexing multiple channels in a given data stream and thereby increasing spectral efficiency of a channel.
Nevertheless, despite these apparent advantages of PM/FM systems, there are fundamental and technological issues with the optical transmitter, the communication channel, and the receiver that have limited the development of this technique. These include:    1) Laser phase noise and spectral instabilities in both coherent transmission/reception processes. The unavoidable phase noise that accompanies the output spectrum of a semiconductor laser causes considerable spectral broadening that appears as baseband noise after demodulation. Although differential detection methods reduce the requirement for laser phase stability, typically, a source with linewidths Δν of a few MHz is required.    2) Intensity noise in the local oscillator. IM-DD is largely insensitive to intensity noise, but coherent detection, owing to the presence of a local oscillator with large power (˜1 mW), is much more sensitive (requiring RIN of −160 dB/Hz or better). Differential phase methods can mitigate this significantly.    3) Drift in the optical frequency of both the transmit laser and LO. This requires the LO to have optoelectronic automatic frequency control loop with a wide acquisition range. Furthermore, homodyne systems require automatic control of the optical field instantaneous phase.    4) Depolarization. Propagation in normal optical fiber leads to signal depolarization that requires a receiver structure with active polarization control or diversity detection.    5) Legacy interface. Virtually all networks today are based on IM-DD technology. Radically different modulation format (e.g. DPSK) and signaling may require significant re-engineering of existing switches, routers and other network components.